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608 ~ta. w. ~. F~An~SrD~S 0.~ Tr~E Gv.O~0OY oF [Aug. 19o 5,

29. On the GEOLOGY of FAWR a~*d MOEL L~.Y:F~A:Sr. By WILLIAm1 GEOR6E FEARNSlDES,M.A., F.G.S., Fellow of Sidney Sussex College, Cambridge. (Read January 18th, 1905.)

[P~w. XLI--MAP.]

l. INTRODUCTION. T~ mountains known as and are situated somewhat to the north of the centre of the county of Merioneth, and lie immediately east of the main water-parting of . The area studied lies wholly within the quarter-sheets 13 S.E., 13 S.W., 21 IN.E, & 21 I~.W. of the 6-inch Ordnance- Survey maps of . Rising from a plateau some 800 to 1200 feet high, they attain heights of 2800 and 2437 feet respectively, and, being entirely above the cultivation-zone, are practically uninhabited. The Great Western ]3ala- Railway crosses the northern end of the district, to which Arenig Station, close under the northern slopes of Arenig Fawr, affords easy access. Although to a certain extent glaciated, the mountains are swept clean rather than Drift-covered, and it is only in the lower ground that moraines become inconveniently abundant. The adaptation of surface-forms to rock-structures is exceedingly complete, and few are the places where the ribs of harder rocks do not protrude through the scanty soil or turf. Being open to the full blast of the westerly and south-westerly gales from the ocean, the district, receives its full share of weather ; and, in consequence, ahhough the eastern slopes are often grass- or heather-covered, the steeper parts of the western slopes are rugged and bare, and afford magnificent exposures of the component rocks. Further, the structure of the district is fairly simple, and the cleavage, though present, is not s(> strongly developed as to prevent one from breaking the rock along the bedding-planes.

:Ever since the time when Sedgwick applied the name of Arenig Ashes and Porphyries to the lower series of h'orth Welsh volcanic rocks, the Arenig district has been a source of interest to geologists. Yet, and notwithstanding that the name of Arenig is so constantly upon the lips of all who study or teach the geology of the Lower Palteozoic rocks, no complete or detailed account of it has yet been published. The only connected general account of the district which I have found, is that contained in the first edition of Sir Andrew t~amsay's ' Geology of :. '~ A good deal of additional information is contained in the second edition of that work (.1881), but is somehow

1 Mere. Geol. Surv. ~ol. iii (1866) Downloaded from http://jgslegacy.lyellcollection.org/ at University of California-San Diego on January 17, 2016

Vo1. 6 I.] ARENIG FAWR AND MOEL LLYFNANT. 609 less easy to extract. Much more instructive are the few pages devoted by ~essrs. A. V. Jennings & G. J. Williams ~ to the Garth Grit, and the recent paper by Miss G. L. Elles ~ on the general pal~eoutology of the Arenig rocks of Wales. Passing now from printed matter, I take this opportunity of acknowledging my indebtedness to Prof. T. McKenny Hughes. F.R.S., and also to Mr. G. J. Williams, F.G.S., H.M. Assistant- Inspector of Metalliferous Mines, of Bangor. Prof. Hughes has, in the course of the last twenty years, made frequent excursions to Arenig, alone and also as leader of large classes of Cambridge students. He has in this way collected a vast amount of ex- tremely useflfl material, and all this, together with the corresponding field-notes and sketch-real)S, he most generously placed at my disposal. Mr. G. J. Williams was long resident at Ffestiniog, which is only about a dozen miles away, and more than twenty years ago discovered the fascinations of the Arenig district as a collecting- ground. Unfortunately, change of residence and increased pressure of work have latterly prevented him from continuing his researches, which have never been concluded; and when I announced my intention of mapping the whole comJtry on the 6-inch scale, he kindly gave me a day over the ground, pointing out fossil-localities. Moreover, since that time he has always allowed me full access to his fine and carefully-localized collection of Welsh fossils.

II. DESCRII'~ION OF THE ROcK-SuccEssioN. The general succession (see also figs. I & 2, pp. 610-11) of the sedimentary rocks of the district is as follows :-- Dieranograptas-Shales. Ga r a d o O. Derfel or OrtMs-Limestone. J Rhyolitic Ashes = Upper ~ Upper Ashes of "~ Massive Ashes = Middle f Arenig. ] Aeid Andesitic Ashes = Lower Daerfawr Shales. Zone of Di@mograptus Murchisoni. ~ Llandeilo. Platy Ashes. ) Lower (Hypersthene-Andesites) ! Great Agglomerate. ~ Ashes or' Arenig. ) Olehfi~ or B~dus-Shales. Zone of Didumograpt~s 9 bifidus. [ Filltirgerig or ttiru~do-Beds. ~ Zone of JDidymograpgus Erwent or Ogygia.Limestone. ~ hirundo. ~- Arenig. ]~enllan or Calymene-Ashes. BasalLlyfnantGrit.~Exte~tsu~-Fiags. t Zoneextensus.OfDid~mograptus )t

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l, Manod & the Moelwyns' Quart. Journ. Geol. Soc. vol. xlvii (189l) pp. 373-74. 2 Geol. Mag. 1904, lop. 199-211. Q. J. G.S. :No. 243. 2 u Downloaded from http://jgslegacy.lyellcollection.org/ at University of California-San Diego on January 17, 2016

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612 ~IR. W. O. FEARNSIDES ON THE GEOLOGY OF [Aug. I9O5,

Amnodd or Shumardia-Sh'ales. Tai-tlerion or Asaphellus-Flags, I Tremadoc. Nant-ddu or Belle~,~2ho~t-Beds. Dic@o~ema-Band. 1 l~obe- and PsiloceThal,us-Beds. { PelCura-Beds. 1 Orlhis-le~tieularis Band. f Black Band. Dolgelly. Paraboli~m-Bcds. Li~lyu{e[[a.Beds. [Ffestiniog. Grits and Flags. J There are Mso important intrusions of andesites (ahnost identical with tho Lower Ashes of Arenig), hornblende-porphyrites, and ophitic dolerites.

The Ffestiniog Beds [22]. The oldest rocks of the district~ occur in the west and south-west, and belong to the shallow-water phase of the Linfpdct-Flags. The) are a monotonous series of grits and flags with hardly any fossils, and determine a belt of most uninteresting, boggy, heather-covered, rolling moorland of only slight relief. Westward they. rise to form the steep escarpment which locally determines the main watershed of Wales. Although usually dipping" eastward or north-eastward at about~ 30 ~ they are so broken up by folds and faults of small amplitude, that, in the absence of any continuous exposure, it is impossible to form any exact estimate of their thickness; but, as the outcrop is considerably more than a mile wide, that, thickness must be considerable. The beds are best. exposed in the valley of the Lliw, where also may be seen a fine large sill of hornblende-porphyrite, quite like the well-known sills which occur at about the same horizon at Dolgelly.

The Linqulella-Beds [21 ], Towards the top these grits and flags become much finer in texture, and in the highest 30 or 40 feet are crowded with specimens of Li~gulella Davisii, M'Coy, which, in places, beconie so abundant as to form the greater part. of certain bands (4(~37). l These beds are exceedingly easy to recognize by their mode of weathering, for, fi'om being bluish-grey, they become first quite rusty and brown and then increasingly paler, until finally the 5" are almost white. A similar belt of fossiliferous flags occurs in the same relative position at Penmorfa (Portmadoc) and at lthobell Fawr ; hence it would seem that the division should be traceable all round the Itarlech dome. The beds are well exposed in the two westernmost branches of the Trinant Valley.

The Parabolina-Beds [20]. Passing upward, the Lb~gulella-Beds become finer-grained and less gritty, giving place gradually to the hard flaggy shales of lhe x The numerals in parentheses throughout tlle text of this paper refer to the numbers of tile correspol~ding r(~ek-specimens preserved in the Sedgwick Museum at Cambridge. Downloaded from http://jgslegacy.lyellcollection.org/ at University of California-San Diego on January 17, 2016

Vol. 6I.i] *RE.~IG rXWR .~XD ~OEr, rLX'FXAXX. 613

Pcwabolina-Beds. These are dark bluish-grey, and give a sonorous ring when struck ; they break with a curious, china-like, conchoidal fracture, and tend to fly before the hammer. They contain a certain amount of iron-pyrites, and coat themselves with yellow rust in the early stages of weathering, although this afterwards disappears and ti~e whole rock appears much darker. In the lower parts they are quite unfossiliferous, but about the middle certain bedding-planes yield indifferently-preserved examples of Par(lbolina ,~pinulosa, Wahl., which becomes abundant in the higher beds.

The Orthis-lenticularis Band [19~. Some 200 feet from the base of the Farabolimt-Beds appears a band of darker and more earthy shale crowded with innumerable shells of Orthis lentic,bwis, 1)ahn. This band is only some 4 or 5 feet thick, but is readily recognized at intervals all round the Harlech dome from Ogof-ddu near Criccieth to Dolgelly. It is best seen in the old Nant-y-Derbiniad slate-quarry, immediately north- west of the are~ mapped, but is also very evident in the western Trinant-stream section, about 100 yards south of the confluence with the eastern tributary. In both these places the Orthis is abundant enough to render the shale quite calcareous.

The Peltura-Beds [18]. Above tile Orthis-hnticMaris Band begins the famous ' Black Band,' distinguishable from all other beds in the district by the fact that when it is scratched with a hammer the streak produced is quite black. The coming-in of the black coloration is of course gradual, and, with practice, one can tell by the streak exactly with what bed one is dealing. Many have thought that the blackness of the beds is due to graphite or organic matter; but a study of the way in which the rock rusts in weathering leads me to think that it is rather due to the presence of finely-divided, slightly- decomposing iron-sulphides. When heated alone, the slates become slightly paler, and acids, which will dissolve iron-pyrites but have no action on graphite, completely bleach small pieces of it. The sediment composing the Black Band is finer than that of either the beds below or above it, and accordingly has, in places, taken on sufficient cleavage to tempt some persons to open trial- holes for slate. The slates obtained are very soft and silky, but they rust and scale at so early a stage in the weathering that, although easy to work, the)" have not proved to possess any per- manent value, and all the trials are now abandoned. The best- cleaved material contains abundant distorted fragments of Peltura scarabceoides, Wahl., and fine, large examples of Agnostus trisectus, Salt., are not uncommon. The lower layers near the Orthis-Band yield occasionally Slfl~ceroiJdludmus cdatus, Bceck, and one or two specimens of Ctenopyge pecten, Salt., have also been found there. Downloaded from http://jgslegacy.lyellcollection.org/ at University of California-San Diego on January 17, 2016

614 xtI~. W. 6. FEAI',.NSIDES ON :l:ttI-: 6EOL06~ OF [Aug. 19o 5,

The best exposure in the district is the stream-section in the lower part of the Trinant Valley : but that in the small stream which comes down from Moel "Lhfnant to the Beudy-uehaf of Hendre Blacn-lliw is also quite good. The Drift south of Tat tterion, too, affords a good collecting-ground, and the specimens of t'cltura which it contains in abundance are the only evidence for the completion of the Li~,g~d,-Flag succession to the east of the great ~orth-al~d-South Fault.

The Niobe-Beds [171 . Towards the top the beds of the Black Band lose their silky ci:araoter, and become cleaved mudstones rather than slates, lu b,~ra is no longer abundant, but occasional examples of Niobe Homfrayi, Salt., and Psiloce]~halus imwtatus, Salt., take its place. Passing up, the amount of pyrites rapidly diminishes, and through dark-blue the sediments assume a rather light leaden-grey colour. Weathered and joint-surfaces are no longer thickly coated with scaly rust, but are merely filmed over with it, and often appeal" iridescent. The character of the sediment also alters, and the main mass of the -Xiobe-Beds is not unlike that of the lower part of the Parabolifla-Beds. The jointiug, however, is quite different, and is rather like that of the well-known ' needle-slate' rocks of Port- madoc, which belong to the same horizon. Judging from casual specimens, obtained from washouts in the heather-covered moorland, the Niobe-Beds must occupy a good deal of ground on the north and wes~ of Moel Llyfnant, but exposures are not encouraging. The best exposure is undoubtedly the stream-section along the eastern branch of the Trinant Valley, which runs along the strike of these beds for more than half a mile, This section well shows the relation between the beds and a series of sill-like intrusions of granular andesite or dolerite, which characterize this horizon right across the district. Similar beds and inlrusions are well seen in the old road-section west of ~ant-ddu, in the railway-cutting north of Y Waun, ~hich is east of the great, North-and-South Fault, and again for about a mile to the west and norih-west of Hendre Blaen-lliw. The Dictyol~ema-Band [16 pars]. Overlying the .35"obe-Beds is the Dict~/o~ema-Band, which (most unfortunately) is even more imperfectly exposed than they arc. Dictyo~e~a has been found abundantly in certain small boulders in the Drift of the Tryweryn Valley and elsewhere, but is only seen in situ at one locality. Fortunately this spot, a few yards west of ~ant-ddu, adjoins one of the few sections from which Niobe has been obtained, and it is perfectly clear that there, as ever in the Portmadoe district, the Diet?fo;iema-Band overlies the _Niobe-Beds, and has no~hing whatever to do with the Black Band below. On this evidence, I would class the Welsh Downloaded from http://jgslegacy.lyellcollection.org/ at University of California-San Diego on January 17, 2016

Vol. 6Io~ ARENIG FAWR AND 3IOEL LLYFNANT. 615

Dictffonema-Band as the lowest division of the Tre- madoc, and would place the ATiobe-Beds as the highest member of the Dolgelly or Upper Lingula-Flags. Wherever tbund, Dictyonema is always enclosed in dark bluish- grey shale, which on inside joints is invariably filmed over with iridescent iron-oxide ; also, it is always associated with spicules of some sponge like Protospon(/ia. As above mentioned, the onh" good exposure of the Dictyonema-Band is the Old-Road section, a few yards west of Nant-ddu; but sponge-spicules in situ and Dictyonema in a loose block have been found close to the mountain- fence, about half a mile due west of the summit of Moel Llyfnant. The thickness of the band is quite small, and, as at Penmorfa, cannot much exceed 15 or 20 feet.

The Nant-ddu or Be!lerol~hon-Beds [16 pars]. Above the Dict!/o~ema-Band is the most monotonous and un- interesting series ill the district. This, in the absence of any more characteristic fossil, I have termed the Bellerophon-Beds. It consists of ~ series of hard, rather dark-grey, gritty shales, usually exhibiting a certain amount of incipient needle-cleavage ; and, in the upper part, where the beds become softer, it has often been prospected for slates. At certain horizons the shales become quite calcareous, and flattened concretions of cone-in-cone chalybite up to a foot or two in diameter are almost characteristic (4638). The lower part of the series has yielded no , but occasionally contains a few oval Lin.q~d~, Acrotreta, and frequent, rather dis- torted, undeterminable Bellerophons. The best exposure is in the banks of the stream of Nant-ddu, and the entrance-cutting to the old manganese-mine which was worked in these beds there. Bellerophons are most abundant iu the Afon Amnodd-bwll, about a quarter of a mile south of Yr Orsedd. A small trial-hole for slate, opened in higher beds close under the overlying Tai-~Ierion Flags in Nant Rhos-ddu, yields also examples of a broad form of Asaphellus Homfrayi, Salt., and a few distorted specimens of Olenus (Parabolinelht) triarthrtts, Call., and O. (P.)Salteri, Call., which suggest a correlation with the lower fossiliferous horizon of Belswardiue Brook, Sheinton (Shineton).

The Tai-Herion o1" Asaphellus-Flags [15]. Immediately above these fossilitbrous slates come the much coarser and more quartzose beds, to which I have applied the name of Tai-Herion or Asaphellus-Flags. These are a well- marked series of hard, blue, shaly or slaty calcareous flags, and contain numerous worm-tracks and castings filled with white quartz-sand (4639). Certain beds among them also show remark- ably well a rather curious form of ripple-bedding. In these 'ripple-bedded' rocks, alternations of coarser and finer sediment take place laterally as well as vertically; and the rock appears Downloaded from http://jgslegacy.lyellcollection.org/ at University of California-San Diego on January 17, 2016

616 ~tR. w. c,. FEAm','SlDES 0X XnE G~OLOGY OF ~Aug. t9o5, to be built up of flattened lenses of the coarser, closely set in a matrix of the finer material. Such rock, when compelled to break along the bedding, affords surfaces which are either nodular, or crossed somewhat irregularly by ripples of short wave- length. Curiously enough, such ripples always appear to have a trend which is almost parallel to the strike of the bedding-planes, which here makes a considerable angle with that of the cleavage. Apart from the ripple-bedding, the Asaphelh~s-Flags are much more massive than any of the other divisions of the Tremadoc; and this character has so enabled them to withstand the agents of denuda- tion, that they stand up as a visible feature right across the district. The same property has also caused them to be considerably quarried for wall-building by the makers of the roads. Fossils are not superabundant, but a good many very large specimens of Asa2)hellns Homfrayi, Salt., of broad tvl)e have been found in the alternations of the coarse and fine layers of the ripple-bedded flags ; and it is noticeable that all such specimens are tbund with the dorsal side upwards, and are filled with coarser and covered by finer material. One or two enrolled specimens have been found, and these too are filled with coarse, and embedded in fine, material. The best and most accessible exposure is that afforded by the road-side quarry immediately west of the Sant-Rhosddu bridge at Tai tterion. The waterfall just below that bridge is determined by the same beds, and the old quarries west of the Amnodd-wen road, near ttafotty Filltirgerig, are in similar though slightly- higher beds. Other sections are also seen at Yr Orsedd, and a curious piece of folding below Ceunant-y-gareg ddu allows them to appear as inliers in the gorge there. The sections west of Moel Llyfnant are not so satisfactory, and have not proved to be very tossiliferous. The whole series is not more than 80 or 100 feet thick. It appears to correspond with the gritty shales of Penmorfa Post-Office, although unfortunately no Dilceloce2halus has as yet been obtained here.

The Amnodd or Shumordia-Shales [14]. Above the Asaphellus-Flags the beds gradually become much softer, and pass into a series of fossiliferous blue-grey mudstones. Except when baked by the intrusive dolerite-sills, which are common at this horizon, these beds are much softer than any of the other beds of the district; and so they usually occur in hollows and boggy places in the moors. Though sheared and considerably distorted, they rarely show any definite cleavage. Close to the dolerite-sills they nre often much hardened, but break readily along a series of close-set quadrangular joints. In such places certain beds take on a very characteristic concretionary or shrinkage-jointing, and weather out into a mass of ovoidal lumps and curved splinters rather after the fashion of a m~ssive basalt. When such con- cretions are broken across during the process of weathering, one finds the successive rings beautifully and differentially iron-stained, Downloaded from http://jgslegacy.lyellcollection.org/ at University of California-San Diego on January 17, 2016

Vol. 6I.~ XI~E.Xm FAWR AND MOEL LLYFNtsr. 617 after the manner of the Moughton Whetstones of Yorkshire. Fossils are, as a rule, fairh" abundant, the most characteristic of them being the beautiful little Shwmardia. Slabs with ten or a dozen specimens of Asaphellus Ho~(f)'ayi, Salt. (of narrow type) are not uncommon, and Aynostus Siedenbladhi, Linn., and Macro- cystella Ma~'ice, Call., may also be obtained from most exposures. The fauna obtained is as follows :-- Asaphelhes Hom./).ayi, Salt. Dikel,),~ephqhes. Sh~ema~'di(e salopieneis (Call.), ef. Uheir~o.tts. celco~diea, 3Ioberg. 6bnu&ria Hom.~'ayi, Salt. Ogggia (Niobe) scuta/rix, S:~lt. Theca ,,l)ercMala, Salt,. Olemts(Pco'a/~oli,el[a) trico'thrus, Call. Belle,',,pho;~ mullistria~us, SMt. Agnostics Siede~bh~dhi, Linn. Cteaodo;d(~. Sympl~esur~ts Crqftii: Call. Liny~d. eL .5)>ko&oni. Call. Re mopleurides. dcr.treta (Oholel&) Sabri.ae, Call. Holometopas. 15"otospo~(qin. spicules. There are also abundant, branching, carbonaceous markings which are often regular enough to suggest graptolites, but are not sufficiently well preserved for identification. The best localities and collecting-grounds are the east-and-west stream-sections south of Amnodd-wen and north of Amnodd-bwll respectively. The latter is particularly good, and has yielded the best specimens of Shumardia and many beautiful slabs of Macro- cystella Mariee, Call. The gorge known as Ceuuant-y-gareg-ddu presents an excellent exposure of the lowest beds, and has yielded examples of all the fossils recorded, except Remople~o'ides. Remo- 2leurides, indeed, seems to be exceedingly rare, and is only repre- seuted by Mr. Wiliiams's one specimen, which came from Amnodd- wen. The best Asal)hellus-slabs were obtained from the bed of the stream, which crosses the road about 250 yards south of Hafotty Filltirgerig. 3_s will be seen from the foregoing list, the Shumardia-Shales may be exactly correlated with the main fossiliferous horizon of Sheinton, and with beds overlying the Post-Office Flags at Pen- morfa. The highest Shuma,'(lia-Beds of the latter locality, which are there exceedingly prolific, do not seem to be represented here, and the overlying Angdiaa-Beds are almost certainly absent, probably by reason of the succeeding unconformity.

Here, then, ends the series which British geologists are in the habit of calling Cambrian, and, in leaving it, I would remark upon the exact correspondence between each of the divisions and its homotaxial equivalent at Portmadoc. Each fossiliferous band, each change in the character of the sediment, can be recognized ; and, although the total thickness must be greater at Arenig than at Tremadoc, I think that the dift~rence is due rather to subsequent packing by earth-movement than to diversity in original deposition.

Above the Shumardia-Beds, Asapkellus-Flags, or Belleropho~- Beds, is a surface of unconformity. This is not a great dis- Downloaded from http://jgslegacy.lyellcollection.org/ at University of California-San Diego on January 17, 2016

618 mI~. w. c. rE, axslmcs ox ~I~J~ GI.:OI~OGror [Aug. I9o5, cordanee marking the uprise, wane, and degradation of some huge continental mountain-chain, hut an unconformity and surface of overlap produced by the passing of some early Caledonian earth- wave, which has left but little permanent impress hi)on the dip and strike of the rocks which it afi'eeted. So slight was the disturbance, that the district mapped is almost too small to permit of proving it; and one might hammer and map for many days on end, without: discovering any discordance at all. It was only by following the line of Mr. Williams's Garth Grit for severa]~ miles to the west, that I was able to make up my mind as t<~ the importance of the unconformity. In that direction, the grit rests successively upon all members of iche Tremadoe Series, and at the Nant-y-De,'biniad slate- quarry comes into the Black Band or Peltm'a-Beds of the Dolgelly, or Upper Li~gula-Flags. Further excursions along the same line suggest that the various members of the Lower Ordovieian Series overlap each other westward against the surface of unconformity; an(/ the knowledge tha~ the similar grit at Ty-obry, Minfibrdd (Portmadoc), is directly overlain by shales belonging to the zone of Didymogra2)t~s M~rchiso~d, Beck, lends credence to such a supposition.

The Basal Grit of the Arenig Series [13]. Upon the unconformity rests a grit, notable for its inconstancy of thickness and variability in character. Sometimes it may be as much as 50 to 100 feet thick, but usually it thins off rapidly, and may entirely die out in something less than 150 yards. Occasionally it is clean and white, and from the abundance of disseminated vein-quartz may become almost a quartzite. More commonly it is rather impure, and has much ash)" material among it. Usuallyit is a fairly-coarse, even-grained grit; but sometimes, especially when thiekest, its middle part becomes almost conglomeratic. In all ea~es, the constituent grains are rounded rather than angular. Grains of hyaline quartz are fairly common, but pebbles directly derivable from the underlying Tremadoc or Dolgelly Beds are conspicuous by their absence. Under the microscope, it is seen that the grit is a good deal sheared, and that grains usually show well- developed strain-shadows and suture-cracks. The quartz-cement is in crystalline continuity with the adjoining grains, the original out- lines of which are often almost obliterated (4640). A certain amount of carbonate-cement (ehalybite ?) is also present, and patches of rather fresh andesine or oligoelase are not rare (4641). No newly- formed minerals were observed. The best exposures are the patches mapped on the northern flanks of Moel Llyfnant and adjoining the dolerite-intrusion south of Llechwedd Erwent; but the grit is more easily visited at the patch east of Waen-goch on the way to Amnodd-wen, or that along the Ffestiniog road at Lleehwedd Deiliog. Downloaded from http://jgslegacy.lyellcollection.org/ at University of California-San Diego on January 17, 2016

Vol. 6i. 1 AI~'IG r.~Wl~ AXD XO~I, LL:~F.X.~T. 619

The Llyfnant or E.cte~zsus.Flags [12~. Upon the Basement Grit or surface of unconformity, as the case may be, comes a distinctly-defined series of well-bedded blue-and- white quartzose flags. These flags are remarkably resistant to the weathering processes, wherefore they determine a prominent feat~lre right across the district, and, being well jointed, give rise to the beautiful stepped or terraced scenery which is so well seen on the eastern flanks of Moel .Llyfllant. They are remarkably-welt bedded, and, being provided with convenient shah- or micaeeous partings, split readily into fine large slabs. In former days these were quarried for flags and hearthstones, which were mucl~ sought after down at Bala. Under the microscope the bedding becomes even more apparent, and it is seen that calcite and shreds of detrita] mica make up quite as large a proportion of the finer bands as do the actual quartz-grains. There is also a great deal of unresolvable muddy paste and some secondary quartz. The quartz-grains are much more angular than those of the underlying grit, and are often also curiously ragged at the edges (4(i42). Although so flaggy, the Llvfnant Beds are yet quite fossiliferous, and certain of their bands ha~'e yielded very numerous graptolites of the extensiform type. The most abundant form is Didqmoy;al~t~,s~ deflexus, E. & W., but Di;l~tmograpt~s e,cte~sus, Hall, is not rare, and Mr. Williams has also specimens of Lo:tcowg~'apt~,s Loga~,i, and a Tetragmptus which is probably Amii. Tails of some large OF~/gia have also been found, and worm-tracks and castings (both small and great)are extremely abundant. The markcdly-flaggy beds are from 150 to 200 feet thick. The best collecting-ground is afforded by the old quarries south of t[afotty Filltirgerig, or by the stream-section which adjoins them. The general character of the beds, however, is better shown in the precipitous north-eastern face of Moel Llyfnant, or in the more shelving slopes of Llechwedd Erwent.

The Henllan or 6alymene.A~hesS [11]. Passing upwards, the well-bedded character of the Llyfnant Flags gradually disappears, and. as the sediment does not become corre- spondingly finer, the succeeding beds assume a notably ' blocky' character. With this the)- also take on a toughness which def{es description, and, although well-jointed on a small scale, appear to retain neither bedding nor cleavage. Their colour is grey, and they are granular and conehoidal in fracture, and in weathering become covered with a characteristic, thick, cream-white crust. They are more calcareous than the underlying flags, and contain much less detrital quartz and practically no mica (4643). They appear to be derived from some more or less distant volcanic source, which, judging by the thickening and increase of ashy material southwards, probably lay in that direction, and provided materials richer in lime (and therefore more basic) than any of the products of the later volcanoes of the district. Downloaded from http://jgslegacy.lyellcollection.org/ at University of California-San Diego on January 17, 2016

620 mR. w. G. FEARNSIDES ON THE GEOLOG~_" OF [Aug. x9o5,

They are not abundantly fossiliferous, but yield occasional large and rather well-preserved specimens of Calyme~e, probably C. pa~'vif~'ons, Salt., which appear only during the la~er stages of weathering. With these also occur a tew large gasteropods, such as .Raphistoma or Maclurea; and many indeterminable Ortho- ceratidm, including a possible Emloco'as. In order to prevent confusion with the underlying beds, the local name is taken from the western slopes of Craig Henllan. There, unfortunately, the exposures are far from good, and much better collecting-ground is afforded by the higher northern slopes of Moel Llyfnant. The structures and general relations are also well exhibited a short distance south of Ceunant-goch and all along the slopes of Llechwedd Erwent.

The Erwent or O qygia-Limestone [10]. Towards the top the Caty,~e~e-Ashes become mixed with a considerable proportion of shaly material, which, being arranged in thin lenticles not more than an inch or two long, gives the rock a streaky-bacon-like appearance. The rock is still very hard and tough, but is now sufficiently calcareous to become porous and breakable when weathered, and in such conditions usually yields quite good casts of shelly fossils. The matrix of the rock still includes much of the basic ash characteristic of the Calymene-Ashes below, but has in addition a good many chips of that acid plagio- clase which is so characteristic of higher beds (4644). The most abundant is now Ogy.qia ,S'elwymYi (Salt.), which must sometimes have attained a length of 8 or 9 inches, although more frequently it is represented by specimens only 2 or 2J inches long, or by numerous tails not more than hzdf an inch across. Tails of this species are much more abundant than heads, and nearly all the moderately-complete specimens are more or less enrolled. Calymene 2)arv(t'ro~s, Salt., is still fairly abundant, and specimens up to 4 inches long have been found. An Ampyx (possibly A. Salte~'i, but very like A. domatus, Linn.) is abundant at certain exposures. Of other tbssils an O,'this like Carausii, Salt., or calligrcm~ma, Dalm., is characteristic of the lower, and Obolella plumbea, Salt., of the upper, part of the series. Orthoce~'as is everywhere, and many-branching dendroid graptolites are occasionally met with in the shaly layers. The entire fauna obtained is as follows :-- 09yffia Se[w!/nii, Salt. BeNeropholt. Ca l!/f,wne parv(f,.ons, Salt. P,/cearca. Ampyx Sal/eri (?) or &~rnatus, Linn. ('te~wd,mta. Orlhvceras e~wrina[c, Salt. Or/his eallisramma, Daim. Or/hooey'as zericcum, Salt. O,'thi8 Carau~ii, Salt. Orlhoceras sp. Ob,lel[a plumbea, Salt. Raphisto~za. Lb~gula I?omm#i, Salt. Maeh~rea. (',l[ograptus radic,!us ? (Hopk.).

It is noticeable that the whole aspect of this fauna, its general composition and numerical proportion of forms, is exceedingly Downloaded from http://jgslegacy.lyellcollection.org/ at University of California-San Diego on January 17, 2016

62I suggestive of the NeswJ.etus-Beds of South Wales. The fossiliferous character of these beds has long been known, and it was from them, at localities nort~h and south-east of Tai Hcrion, that, the officers of the Geological Survey obtained all their truly-Arenig specimens. A better collecti~kg-ground is afforded by the slopes of Llechwedd :Erwent, and bv those between that hill and Foel Boeth. The highly-fossiliferous band is not more thau 10 or 12 feet thick, but is rendered very evident by the streaky eharact,er of the rock. The outcrop of the belt runs roughly parallel to the contours of the hill, from the northern dolerite-iutrusion of Llechwedd Erwent to the pass which goes over to the Llafar Valley, and then turns southward following round the slopes of Foel Boeth, and is fossili- ferous all the way.

The Filltirgerig or Hiru~,do-Beds [9]. Becoming still more streak)', the Erwent Beds pass gradually, and finalIy rather suddenly, into the dark, block?', calcareous shales of the Hirundo-Beds. These, too, are as a rule slightly ashy, and in the upper part include one or two well-defined bands of tuff. In the main, however, they are of detrital origin, and contain a great deal of remarkably-angular chips of quartz, together with some very well-preserved flakes and shreds of eolourless mica (4645). The more shalv beds are well bedded, and show rapid alternat~ions of sandy, shaly, and micaccous material, as in the corresponding beds of Pont SeionL Fossils are not especially abundant, but graptolites, where found, are usually preserved in pyrites and stand out in full relief. In shaly beds Didymograd~tus hirundo, Salt., is most frequent, and with it small forms referred to D. nitidus, Hall, or D. pat~dus, Hall, have been found. There are also a good many ill-preserved Tetragraptids, including Tetra- gr(~tus serra, Brongn., and T. recliaatus, Elles & Wood; and a single example of Did?fmograptus gibber,lus, Nieh., has been found. These are associated with imperfect tYagments of Asaphid trilobites and oflEglin~t. In the very streak) beds at the base of the series Obolella plumbea, Salt., continues to be abundant, and with it has been found a fine slab of Azjyoy~.aptus s~ecicus, Tbg. When traced across the district these beds undergo considerable variation. In the south the streaky character is more or less constant throughout the series, but in the north the middle part becomes less calcareous and more shaly, and at the top there appear two well-defined as]t-beds, which in the extreme north attain a thickness of quite 20 feet. Under the microscope it is seen that these ashes agree in composition with the andesites of the main Lower Volcanic Series of Arenig (4646 & 4647). The lower band contains numerous well-rounded grains of quartz and certain ' nummulite '- like pebbles of friable shale which agree exactly in character with the masses described by Prof. Grenville A. J. Cole ~ as occurring

1 Geol. M~:g. 1890, p. 449. Downloaded from http://jgslegacy.lyellcollection.org/ at University of California-San Diego on January 17, 2016

622 ~. w. G. 1,'EAI'~'SIDES OS THE GEOLOGY OF [Aug. I9O5, in the basal Grit of . The upper band of ash is thinner but even more gritty; and it must, I think, have been the character of this rock which led Miss Elles ~ to conclude that the Hirundo-Beds at ~N'ant Rhos-ddu rest unconformably upon Tremadoc rocks, and to record ashes as occurring in the latter group. Above the higher ash the beds are again very block)', and so calcareous that they were termed limestones by Miss Elles, who obtained Didymog~vpt~,s hirumto, Salt., from the layer directly above the ash-band. In general, the Hirwn~lo-Beds are much softer than the Erwent Beds below, or than the volcanic group which covers them with screes fi'om above. Exposures are correspondingly unsatisfactory, and it was only where a small landslip had bared a scar on the slope of Maen Grugog, close to Filltirgerig, that I was able to see the main mass of the shales in situ. Fortunately, in that same area the trend of the glaciation coincides for some distance with the outcrop of the beds, which have therefore pro- vided a moderate proportion of the smaller boulders in the adjoining Drift, and over all the slopes above Filltirgerig examples or' Didyrnograt)tus hi,'u~do, Salt., mav be occasionally found in the Drift. A very good place to search is the Drift exposed in the banks of the Nant Filltirgerig, immediately above the waterfall. The ash-bal~ds and higher beds are best seen in the Nant-llhos-ddu section, but are also easily made out where the)" cross the :Hafotty- Filltirgerig stream. The well-klmwn m~ss of Ampy.c-bearing streaky bed~ there exposed is probably a fallen block or transplanted boulder, and is quite out of place.

The Olchfa or Bifid~,,s-Shales [-8]. Upwards the Hirundo-fauna seems to die out suddenly, and within a foot of the bed in which the last Didymograptus hirundo was collected, quite a selection of species belonging to tl:e Bifidus-fauna may be found. Correspondingly the sediments lose their calcareous character, but this variation is gradual ; alld were it not that the B~dus-Beds are pyritous instead of calcareous, the two would be indeed difficult to distinguish. :Here, however, fossils come to our aid, and are so abundant that even poor exposures will usually yield determinable graptolites. Under the microscope it is seen that the BiyTd,:s-Shales are far finer in texture than those of the Hir,u, do-group, but they also contain chips of deh~tal quartz and shreds of mica which are much too coarse for the rest of the materials. The slide also shows many ragged flakes of what is probably graphite, together with the more usual pyrites (4645). The Bifidus-Beds, like the soft shales of the Shurnardia-group, never seem to be properly cleaved, and, although distorted, the rock always breaks along the bedding-planes, whicb, under the micro- scope, appear .~lightly puckered. The fauna obtained is the typical

1 Geol. M~,g. 1904, p. 207. Downloaded from http://jgslegacy.lyellcollection.org/ at University of California-San Diego on January 17, 2016

Vol. 5 i.] ARENIG FAWR AND MOEL LLYFNANT. 623

Lower Llanvirn fauna of the late Dr. Kicks, but is without the Placoparia and other trilobites common in South Wales. The Bifidus-Beds are most unusually thin, but in their 20 or 30 feet bear record of a very notable evolutiou of graptolite-life. Near the base the commol~est species is Didymog,'aptus namts, Lapw., together with D. pat~das, Hall, Crypto:p'a2)tus t~'icornis, Carr., a Dendroffra2)tus , and Didymoff,'ai)t~s bifi(lus, Hall, of the normal small type. At the top, the vaguely-defined Diplogral)tus dew,tutus, Brongn., which probably includes several species, is most abundant, and with it are Did!/moT~'aptus art~es, E. & W., D. acutidens, Lapw., D. stabilis, E. & W., Glossoff~'apt~s sp. and Crffl)tO.7~'aptus b'icornis, Carr., and many specimens o~' Didymogl'aptus b~(l~ts, ttall, of a type intermediate between D. bifi'dus and D. M,rchisoni. At this horizon also tails and other fragments of some Ogygia, like O. l)eltata, Salt., or possibly 0. Buchii, Brongn., are not infrequent, and a Dionide closely similar to those fi'om Ty-obry has been observed. Like the Hir~tndo-Bcds below, the Bifid~s-Bcds are not well exposed. Coming, however, immediately beneath the great, volcanic group, they have been traced more or less continuously all along their outcrop. In the moorland between the Ffestiniog Road and Nant-yr-Olchfa, they appear upon their dip-slope and occupy a considerable area. Where they just cross the burn the higher beds are exceedingly fossiliferous, and the Drift of the whole area teems with fossils. Under the slopes of Maen Grugog by Filltirgerig they are again well seen, and yield a profitable return of the lower graptolites. The Nant-Rhos-ddu section has been carcfhlly de- scribed by Miss Elles) There the whole series is seen to be much mixed with pyroelastic felspar, and includes also some bombs of ash, a fact of considerable interest in determining the age and focus of origin of the overlying volcal~ic group.

The Volcanic Series. Upon the B~dus-Shales come the earliest members of the great volcanic group of Arenig, the Ashes and Porphyries of Sedgwick. This is a series of very great thickness, but, owing to the almost complete absence of fossils, it can only be subdivided by complete field-mapping, according to the petrological character of its component rocks.

The Lower Ashes and Agglomerates [7]. The earliest of the eruptive products of Areldg is all inconstant series of coarse, well-bedded and platy ashes. These, in the course of some 30 feet or less, pass almost imperceptibly into the massive agglomerate which fl)rms the great bulk of the Lower Series. In places, the Lower Plat)" Ashes seem to thin out, and especially in the neighbourhood of certain of the contemporaneous intrusions to be described hereafter (pp. 62S-30), the Agglomerate comes to rest directly upon the shales beneath. i Geol. Mug. 1904, pp. 205-207. Downloaded from http://jgslegacy.lyellcollection.org/ at University of California-San Diego on January 17, 2016

624 ~R. w. G. r~A~'SlD~:S O-~ ~n~. G~OLOG:~O~" ~Aug. I9o5,

The great bulk of both the Platy Ashes and the Agglomerate consists of shattered blocks and particles of a hypersthene-andesite exactly similar to that of the intrusions to be described hereafter. The blocks of the Agglomerate are set ill a matrix of smaller particles of the same material, and the whole mass is usually to a considerable extent secondarily silieified (4649-51). Most of the blocks in tile Agglomerate are about the size of a hen's egg or of a cricket-ball, but sometimes quite large masses arc seen. With these there are also, especially in the northern district, various large and small masses of crushed and hardened, but uncleared, shale, and in one of these several nice specimens of Lingulella Davisii were found. They came from a shale-mass several feet long, which occurred in the Agglomerate about 100 yards to the west of Rhyd-y-fen. Small pieces of a coarse granophyre, with patches of centrically-arrangcd quartz-orthoclase eutectic, and idiomorphie oligoelase, have been found in the lower part of the Agglomerate, and are interesting as being unlike any of the other rocks of the district (4652). Masses of some ophitic rock, rather like the later dolerites, occur in the higher part of the series. The whole of the Agglomerate is exceedingly massive, and might well belong to a single phase of deposition. It is much thicker in the north and east than in the south, and particularly the south-west, where, though traceable on Moel Llyfnant, it appears to be rapidly dying out. Upwards the Agglomerate passes into the Upper Platy Ashes, which are similar in every respect to those below and equally uninteresting. The thickness of the Agglomerate about Clogwyn Maen Grugog must be something between 300 and 400 feet ; that of the Upper Platy Ashes may be anything up to about 150 feet, and seems to be greatest where the Agglomerate is thinning, as to the east of Craig- y-Bychan.

The Daerfawr Shales [6]. Upwards the Upper Platy Ashes become continuously finer, until they merge into the peculiar series of pyroclastic mudstones to which I have applied Mr. Williams's name, the Daerfawr Shales. These are a most variable series of deposits, and are more in- constant in thickness than even the Basal Grit. Though chemically almost pure eruptive rocks they are often beautifully laminated, and seem admirably adapted for the preservation of fossils. Where thin, they are often quite well-cleaved, and have been prospected for slates in numerous localities. Where thick, they appear uncleared, and include certain sandy beds, with a certain amount of angular chips of detrital quartz and a few mica-plates. In such beds worm- and crustacean-tracks are quite abundant, and it was from these that Prof. Hughes obtained the problematical Saccocaris nmjor and Sacco- earls mi~or. Some of the beds show very fine examples of infiltered iron or other colour-staining, which seems to be later than the inci- pient cleavage and jointing, through the cracks of which it has often Downloaded from http://jgslegacy.lyellcollection.org/ at University of California-San Diego on January 17, 2016

Yo]. 6i.~ ARE2N'IG FAWR &.ND MOEL LLYFNANT. 625 come. The forms taken by this colour-staining are most fantastic, and in strata with the bedding-planes undisturbed may show what appears to be a most advanced type of folding or overfaulting. A_ characteristic mass of pisolitic iron-ore, like those of Tremadoc and Cader Idris, appears along a crush-belt in these shales, and is exposed in the western branch of the Filltirgerig stream, south of Maen Grugog, but the field-evidence is not sufficient to show whether the mass is really a contemporaneous bed or a fault-rock deposit. Of fossils, Diplograpt~s foli ctceus, Murch., var., Diplogra~tu~ angustifolius, Hall, and Dicello(/raptus mo~atensis, Carr., are the only species definitely identifiable. Unfortunately, although these strongly suggest the higher par~ of the zone of Did!/mogmpt~ts .lIurchisoni, they only make us long for more, and do not definitely determine the horizon. With them occur several forms of D~cra~ograTtus and Climacoflraptus, also a small species of lamelli- branch and a very small Obolella. The upper part of the series is again mixed with coarse ashy material, and the passage into the Upper Ashes is quite gradual. The distribution of the Daerfawr Shales on the map is most curious. In part, at least, this is due to the great variations in thickness mentioned above, and may, therefore, depend to some extent upon the irregularities of the original surface of deposition. Largely, however, it is determined by peculiarities of surface-contour, and by the adjustment cross-folds which affected the rigid beds above and below it during the production of the cleavage of the district. The Daerfawr Shales are usually well-exposed. Good sections are afforded all along the base of the Daerfawr precipice. Fossils have been obtained immediately west of Clogwyn-y-Fran, close to Arenig :Railway-station, all along the screes under Craig- yr-Hyrddod, and on the south side of the Amnodd-wen Fault, where the shales are finely colour-banded. The great spread of Y Merdd is not well exposed.

The Upper Ashes of Arenig. The Upper Ashes are not sufficiently fresh to be of surpassing petrological interest. They are the most resistant rocks in the whole series, and determine all the higher summits, precipices, and climbing-ground in the neighbourhood. Dipping with the other rocks gently to the eastward, they have a very wide outcrop along the eastern slopes of the hills, and at first sight appear to be much thicker than they really are. Lower, Middle, and Upper divisions have been tentatively distinguished in the mapping, but there are no very sudden ebanges in character of the material, and what may appear to be good mappable lines at one place usually lapse into a maze of similarities within a re1T few hundred yards along the ~)utcrop.

Q. J. G. S. ~'o. 243. 2 x Downloaded from http://jgslegacy.lyellcollection.org/ at University of California-San Diego on January 17, 2016

626 ~1I~. w. 6. FEAR~N'SIDES 0~ THE OEOLOOY OF [Aug. I9O 5,

Tile Lower or Andesitic Ashes of tile Upper Series [5]. As above remarked, the Upper Daerfawr Shales merge imper- ceptibly into the Lowest Ashes of the Upper Series. This division is more basic than the rest of the series, and generally includes rather abundant shale-fragments. It is often agglomeratic (4672- 73) and well bedded, and may include several thin lava-flows, but none of these extend over any considerable area. In general, it is less resistant than the Middle or Massive Series, and, being well jointed, usually forms the steep faces of cliffs and precipices which are capped by that series.

The Middle or Massive Ashes of the Upper Series [4]. The Middle division of the Upper Series consists of ashes, which are, on the whole, much finer and more altered than those either above or below. They are, however, almost devoid of bedding, and, being jointed on an extremely-large scale, weather out into very massive and imposing forms. In the upper part they have taken on a sort of incipient cleavage-jointing, and consequently often deter- mine rough and almost pinnacular crags. In lower ground they weather most curiously to a greenish cream-coloured rock, with abundant streaks and irregular veins of some pale chloritic mineral which suggests pinite. Concurrently with this form of alteration there seems to be a great tendency to segregation and oxidation of the manganese which is present throughout the rock, and in former days manganese was worked, or prospected for, at frequent intervals all over the outcrop. The manganese obtained was in the form of botryoidal pyrolusite, and it is always said that the ore invariably dies out in depth. Chemically the rocks of this series are intermediate between the acid andesites of the Lower, and the basic rhyolites of the Upper, group. They are closely similar to many of the felsites and felsitie ashes of .

The Upper or Rhyolitic Ashes of tile Upper Series [3]. The Upper division is even more vaguely defined than the others. It consists of ashes more distinctly bedded than those below, and its members usually contain recognizable chips of felspar-crystals. The highest members of the series become so well bedded that they appear almost like massive flags in their weathering. The lower part of this Upper division, and, to some extent, the upper part of the Middle also are frequently almost impregnated with ramifying quartz-veins. In other places apparently-similar beds have been silieified along irregular veins, until they appear like cherts or adinoles (4674). For a long time I was unable to understand the nature of the alteration ; a series of silica-determinations, however, at once set the matter at rest, and showed conclusively that in the more basic members of the Ash Series chert-like veins take the place of the quartz-veins of the acid members. The same series of Downloaded from http://jgslegacy.lyellcollection.org/ at University of California-San Diego on January 17, 2016

Vol. 6i.~ XR~IG rAWR a~ro atOEL l;r.Yrxx~r. 627 analyses showed me that, while the lower members of the Ashes of the Upper Series contain only about 63 per cent. of silica, the higher members may contain as much as 72 per cent.

The Derfel or Orthis-Limestone [2]. Although so thick and so continuous, the Upper Ashes of Arenig end off quite suddenly, and through the massive flaggy ash of the top of the Upper division pass into a few feet of slightly-ashy shale which in turn gives place to a richly-fossiliferous , monticuliporoid, and eystoid limestone. From the fine exposure in the gorge below the waterfall at Pont Aberderfe], I have called this the Derfel Limestone. It should be considered rather as the basal member of the succeeding belt of black shales than as a member of the volcanic group, for, although intensely pyritous, it contains practically no ashy material. When I first met with it in the valley above Pont Aberderfel I was inclined to think of it as a Drift-carried or faulted-in mass of Bala Limestone, but further work has shown that this is impossible. When traced southward, the Limestone, which in the Derfel gorge is 3 feet thick, seems to die out, and beyond Pistyl Gwyn, where it is 1 foot thick, has not been found. Northward, however, it thickens, and is readily mappable along the Valley of the Gelyn, as also about the Trausnant farm in the Valley of the Serw, and north of . The fossils obtained by the officers of the Geological Survey at ' Garn, east of Arenig,' also seem to come from the same horizon cropping out east of Gylchedd. In all these places the limestone rests close down upon the highest volcanic beds, and there can be little doubt that it is conformable with them. The fauna hitherto obtained is as follows :~ Trinucleus concentricus, :Eaton. Orthis (.Dahnanella) un.quis, Sow. Lichas laxatus, M'Coy. Lepta~na rhomboidalis, Welch. Phacops bimuc,'o~zat~z ? (March.). 2)te&ambonites quinquecosta, M'Coy. Cybele verrucosa ? (Dalm.). .Plectambonites sericea, Sow. Acidaspis. 191ectamboniles transversalis, DaM. Illa~nus. Triptesia insularis, Eichw. Cal~/mene. Triplesia spiriJbroides, /~['Coy. Harpides. S[rophomena antiqua, Sow. ~aphistoma a~qualis, Salt. Slrophomena expansct, Sou'. Clitambonites aseendens, Pander. Orthotetes pecten, Linn. Ort his Actonia~, Sow. Olyptocrinus basalis, M'Coy. Orthis(Platystrophia) biforata, Sehlot- Mo-nticulipora lycoTtodon, Hall. heim. JIonticulipora f~rosa, Goldf. OrHds calligrarama , Dalm. Ischadites micropora, Hall. OrtMs (Dalmanelga) deffangula, Dalm. t~itodictffa dichogoma, Portl. Orthis plicata, Sow. _Ptilodictya ex'planata, M'Coy. Orthis sarmentosa, M'Coy. Plilodictya fucoides, M'Coy. Orthis(Dalmanella) testudinaria,Dalm.

What the exact horizon of this fauna is I am unable to make out, for all the , which are the sole really-common well- preserved fossils, belong to species with very long ranges, and might 2x2 Downloaded from http://jgslegacy.lyellcollection.org/ at University of California-San Diego on January 17, 2016

628 ~ta. w. ~. FEXm~SID~S O~ XHr Gv,Or.OrY Or EAug. I9o 5, come from anywhere between the Llandeilo and the Bala Lime- stones. The specimens, however, are smaller than is usual at the latter horizon, and forms transitional between pairs of related species are more frequent than forms distinctly referable to those species. The trilobites are not so abundant, and are but rarely well- preserved. Those identifiable are indicative of the higher part of the Lower or of the Middle Caradoe horizon. On the whole, the fauna seems more closely related to that of the various calcareous grits of the :East-Shropshire Caradoc Series than to any single series with which I am acquainted ; but, in comparing it with those faunas, it is noteworthy that all such large Orthids as Orthis flabellul~tm, O. ves2ertilio, etc. are as yet undiscovered. The commonest fossils in the limestone are a rather small form of Orthis biforata, Sehloth., O. Actonice, Sow., O. testudinaria, Dalm., and Plectambo~dtes sericea, Sow. Monticuliporoids, bryozoa, and stems and ossicles of crinoids or cystoids are also exceedingly abundant, and make up a very large proportion of the rock.

The Dicranograptus-Shales [1]. I have not seriously worked out the beds above the limestone, but what I have seen of them does not encourage me to continue. They are an enormous series of soft black shales or slates, with a varying degree of cleavage. They are beautifully exposed along the Nant Hir and other streams which flow down from the dip-slopes of the Upper Ashes to Bala Lake, along clean sections 3 miles or more in length; but, although I have passed along these daily for several weeks, I have never been able to note a single outstanding feature which would reveal to me the dip-strike or bedding of the rocks, and, except for some ill-preserved ttartfell Dicranograptid~e in Drift-boulders, I have never seen so much as a fossil in them.

The Intrusions. The intrusions of the district belong to two main and two subsidiary petrological types, and must be referred to, at least, two quite distinct periods of eruption. Each occurs in sills spreading along the bedding rather than as dykes, and all are remarkable for the way in which they hold to some fairly-definite stratigraphical, or possibly hydrostatic, horizon right across the district.

The Hypersthene-Andesites [23]. The oldest series of intrusives was erupted at a period contem- poraneous with, or closely subsequent to, the time of the formation of the Lower Ashes and Agglomerates, and like them may be termed hypersthene-andesites. They are volcanic rather than hypa- byssal, amygdular, and probably at one time were only cryptocrys- talline and perhaps glassy. With the Lower Ashes and Agglomerates they have long been known under the name of ' Arenig Porphyry,' and are now being quarried at Arenig Railway-station as 'Arenig Downloaded from http://jgslegacy.lyellcollection.org/ at University of California-San Diego on January 17, 2016

Vol. 6I.] ARE~IG FAWR A~'D 310EL LLYFh'.~.~'T. 629

Granite.' In composition they agree exactly with the Lower Ashes and Agglomerates, as also with many of the lavas and sills of the Stapely Volcanic Series of West Shropshire, but are more thoroughly silicified. In general, the intrusions take the form of rather thick sills, and occur above or below the Great Agglomerate. They are also frequent among the Platy Ashes above the latter, and, as shown in the map (P1. XLI), extend downwards almost to the L]yfnant or Exte~sus-Flags, and upwards as far as the Daerfawr Shales. In several places, as at :Bryniau Poethion or about Llyn Crafanc, actual transgressions can be made out. The thickness of the sills varies enormously, but even the centre of the largest sill which is quarried at Clogwyn-y-Fran close bv Arenig Railway-station, and is quite 200 feet thick, has spasmoclic large vesicles; and smaller sills, which have lost the filling of their amygdules, frequently appear quite spongy. All the sills are more or less columnar in their jointing, the thicker almost as much so as the basalts of the Giants' Causeway. The best columns seem always to belong to the under, and not to the upper, surface of the sills. The question of the intrusive or extrusive origin of the sills has been an open one ever since the time of Sir Andrew Ramsay, who was unable to make up his mind on the subject, and it is only after a good deal of work that I have been able to conclude that they are in the main intrusive. Even now, I picture some submarine volcanic cone sealed between successive eruptions by the chilling action of' the sea, until eventually it became easier for the welling lava to spread itself' among the accumulated rubbish than to burst asunder the cone and pour out over the surface. There is no corresponding set of intrusions among the ashes of the Upper Series, which have suffered far more contemporaneous decomposition than the members of the Lower. For this, and for other reasons, I am inclined to regard the Upper Series as, in part at least, a series of suba6rial accumulation. The total mass of the andesite-intrusions must be enormous, and is quite as great as the whole of the rest of the Lower Volcanic Series. Like the Great Agglomerate, the intrusive sills are always thicker and more abundant in the north-east than in the sou~h- west of the district, Under the microscope, it is seen that the rocks were formerly andesites, but that they have undergone a great deal of subsequent silification and other alteration. The sole really-fresh original minerals are the felspars and the apatite. The former are beauti- fully preserved, and are mostly oligoclase or andesine, usually with narrow extinctions and showing Baveno or window-twinning, in addition to the ordinary albite-twinning. Sometimes the crystals are beautifully formed, while in others the growth has been checked at the skeleton or spongy stage. A certain amount of orthoclase is occasionally met with, but is not especially abundant. The apatite is extremely characteristic, and occurs in elongate hexagonal needles which are unusually large for so fine-grained a rock. Downloaded from http://jgslegacy.lyellcollection.org/ at University of California-San Diego on January 17, 2016

630 mR. w. ~. FEAm~SlDES ON THE 6EOLO6t" OF [Aug. I905,

~Ferromagnesian minerals are, so to say, rather at a discount, and are as a rule represented by ramifying veins and good pseudomorphs of spherulitic pale-green chlorite, which is probably delessite. Some of the pseudomorphs are exceedingly-well formed, and, from their constant octagonal cross-section and elongate outline, are almost certainly after a rhombic pyroxene, which was probably hypersthene. The groundmass of the rocks is always recrystallized and probably always silicified ; when well preserved it shows a parallel or variolitic arrangement of rather stumpy microlites of plagioclase set in a coarsely mieropoecilitic or granular mosaic of quartz or untv~inned felspar (4653-54 & 4657, 4659). Half-digested frag- ments of some ophitic andesine-bearing rock, quite like the later dolerites of the district, are frequently- enclosed in this rock as in the corresponding Stapely rocks ot' Shropshire (466]). The infilling of the larger vesicles is usually calcite, that of the smaller ones is sometimes calcite, but more frequently quartz or spherulitic chal- cedony. Some of the cavities have both quartz and calcite, together with a little chlorite ; and, in these, it is worthy of note that the calcite is usually external to the other minerals (4655-56 & 4658, 4660). The Andesitic Dolerites [24]. Just as the hypersthene-andesites tend to follow the horizon of the Great Agglomerate, the andesitic dolerites spread themselves abundantly about the horizon of the unconformity and the Basal Grit. Unlike the andesites, however, they range in a series of minor masses far down into the Cambrian, and in two localities extend upwards as far as the Great Agglomerate, in one place entering the Daerfawr Shales. Being of a rather basic character, they are usually much cbloritized and very far from fresh, yet petrologically are quite interesting. They are always non-porphyritic, and generally to some extent ophitic. They contain from 5;5 to 56 per cent. of silica. Though occasionally slightly vesicular along their edges, they are distinctly hypabyssal in the mass of the rock, and raE~ge almost to gabbros in the centre of the larger laccolites. Vesicles, when present, are always filled with calcite (4664). Although I suspect, I have not been able to prove, chemical differentiation between the centres and edges of masses : such variations in silica- percentages as do occur seem irregular in their distribution. Under the microscope, it is seen that the rocks belong to a rather acid type of dolerite, and that the ophitie structure is by no means constant in them. The great bulk of the rock consists of a plagioclase-felspar with an extinction-angle ranging on symmetrically-cut albite-twins up to 20 ~ but rarely higher. On powdering the rock, it was found that nearly all the felspar floats off in a mercury-and-potassium iodide solution of specific gravity 2"68, sinking again when the specific gravity is reduced to 2'64. Cleavage-fragments so obtained showed extinction-angles never greater than 8 ~ and mostly gave only 1 ~ or 2 ~ upon the second cleavage. The felspar is, therefore, almost Downloaded from http://jgslegacy.lyellcollection.org/ at University of California-San Diego on January 17, 2016

Vo1. 6I.] AIRENIG FAWR AND MOEL LLYFNANT. 631 certain]y andesine; and as the crystals are mostly elongate and have a parallel or roughly-variolitic arrangement, the rock may be termed an andesitic dolcrite. The augite of the rock is in somewhat smaller proportion than the felspar: it is a pale yellowish or greenish variety, shows bright yellows between crossed- nicols, and has the usual extinction about 34 ~. It, too, is frequently quite idiomorphic, especially where fresh, but in other cases may enclose the andesine ophitical]y. In addition to this, there is also a small proportion of another, still more colourless, pyroxene, with higher refractive index, which gives greys between crossed-nicols and has almost straight extinction. This is probably a hypersthene with a wide angle between its optic axes, and is extremely like the hypersthene of the Tremadoc sills, as also that of the dolerites of the Shelve country. Ilmenite, in networks of skeletal bars, and apatite, in minute elongate needles, are also notable, as at Pen- maenmawr. As alteratiou-products, chlorite of the form delcssite, calcite, and chalcedonic quartz are only too abuudant, and pale and dark epidotes occur (4665). The delessite is remarkable for the way in which it ramifies between and around the best idiomorphic felspar and augite-crystals, and during the weatherirlg gives a sort of pseudo-ophitic a~pect to rock which was even once panidiomorphic. Frequently it also forms good well-shaped pseudomorphs after the elongate octagonal crystals of hypersthene. The quartz present is nearly all chalcedonic ; and although I have sought for, I have not been able to find, any micropoecilitic or eutectic intergrowths between it and the felspar (4662-63). From the fact that the andesitic dolerites enclose pieces of Shumardia-Shale with undistorted fos,ils, and that the slightly- hardened shales of that series which adjoin them are entirely un- cleaved, one must conclude that the date of intrusion was prior to the cleavage of the country. From the way in which the intrusive sills seem to spread themselves in the neighbourhood of certain faults, one might think that their intrusion was connected with the for- mutton of some of these. As wilt be seen from the ibregoing descrip- tions, the andesites and the andesitic dolerites appear to be closely related ; and as the former, which-are also the older, contain half- digested fragments of the latter which are newer, both would seem to have come from the same subterranean magma-basin. In this, as in all other particulars, the andesitic dolerites closely resemble the Shropshire dolerites which, as shown by Prof.Watts, ~occasionally cut strata of Llandovery or even Wenlock age, and I have every reason to think that all are of the same general age.

Other Intrusions [251 . Of other intrusions I need only mention the curious granular andesites, which are so common at the horizon of the Niobe-Beds, and the hornblende-porphyrites, which are seen intrusive among the Middle Lingula-Flags (Ffestiniog Beds) to the west of Blaen-lliw. 1 Prec. Geol. Assoc. vol. xiii (1894) pp. 339 et seqq. Downloaded from http://jgslegacy.lyellcollection.org/ at University of California-San Diego on January 17, 2016

632 ~lR. w. G. rE.~I~SIDES O~~ Z~E GEOLOGX"OF ~Aug. 19o 5,

The granular andesites occur in a series of rather small sills, which seem to be arranged en 6chelon along the bedding- planes among the :Xh'obe-Beds. They contain just a few rounded phenocrysts of oligoclase or orthoclase, but are, on the whole, non- porphyritic. They closely resemble the groundmass of andesites of the main series, but are even more thoroughly recrystallized, and show quite a coarse-grained mosaic of micropoecilitic felspar. The original microlites are short and rather stumpy, and appear to show good flow-structure. The microlites are arranged without any reference to the micropoecilitie patches, and frequently lie across the boundary between two of them (4666-68). On weathering, a great deal of calcite is formed between the micropoecilitic patches, and this, dissolving away upon the surface, gives the rock a very characteristic sugary appearance in the field. The hornblende-porphyrites are only represented in this district by a single sill, but as this is exposed upou its dip-slope, it occupies a very considerable tract of ground, which unfortlmately, however, is mostly covered with Drift. Where seen, the rock seems very like the more basic porphyrites of Rhobell Fawr,' and contains delightful porphyritic crystals of green hornblende and well-formed Carlsbad twins of a plagioclase-felspar (4669-71). Unfortunately, no specimens fresh enough to slice are as yet to hand, and a further consideration of this rock must therefore be postponed.

The Cleavage. Considering its geological position in the midst of the greatest slate-producing region of the world, the district of Arenig Fawr and M:oel Llyinant is remarkably free from cleavage. V~hen one follows the outcrop of the various series to the south or to the north- west for even only a few miles, one is struck by the notable increase in the intensity or:the cleavage; and hence, though not able to bring forward much evidence, I am inclined to think that the absence of cleavage here is due to the accident of ti~e district being, as it were, the keystone of the great Harlech dome. Further, I find that the cleavage of North Wales, as a whole, has a Caledonian or north-easterly to south-westerly trend, and that the average strike of the rocks of the Arenig area is transverse to this. I should therefore suppose that the great masses of unyielding and uncleared igneous rocks of the district may have transmitted earth- pressures along their length without suflbring much deformation, and so may, for a time at least, have held off the stresses from the adjoining softer rocks. I notice also that there occur in the area no rocks the trend of which is now exactly north-west to south-east, but that all strikes change abruptly at a series of faults from north- and-south almost to east-and-west. Such faults I regard as marking the stage when the rigid igneous rocks could no longer stand the strain, and so gave way, as it were, with a rush ; and I should like 1 G. A. J. Cole & T. H. Holland, Geol. Mug. 1890, p. 452. Downloaded from http://jgslegacy.lyellcollection.org/ at University of California-San Diego on January 17, 2016

Vol. 6I.~ ARE~*IG FAWR AND MOEL LLYFNA~T. 633

to explain the rucking-up, local thickening, and pinching-out of such soft beds as the Daerfawr and Bifid,ts-Shales as due to the adjustments between the rigid masses consequent on local pressure- reliefs produced during the process. As to the age of the cleavage, we have already seen that it is later than the intrusion of the andesitic dolerites. It is, therefore, probably post-Silurian; and as, from the relations of the Carbo- niferous, it must date from long before the Carboniferous Limestone, we may tentatively refer it~ to the age of the Old Red Sandstone or Devonian. The Structure. The general structure of the district is that of a terraced escarpment, broker~ both transversely and longitudinally by faults. The scarp as a whole is determined by the Volcanic Series described, and extends more or less continuously right round the Harlech dome, from Tremadoc by the Moelwyns, Maned Migtmnt, the , the Arans, and Cader Idris, to the sea again near Towyn, the &renigs being the easternmost outpost of the ring. As has been indicated, the Massive Ash of the Upper Series forms the culminating ridges of both Arcnig Fawr and Moel Llyfnant, which have also their eastern slopes-largely determined by its dip. A prominent terrace or secondary scarp is determined by the Andesites and Agglomerates of the Lower Volcanic Series, and runs round high up on the western flanks of both mountains. About Craig-y-Bychan, Maen Grugog, and Craig Henllan this feature so spreads out as almost to determine separate mountains. The Llyfnant :Flags, and with them the andesitic dolcrites, give rise to broken ground at lower levels, but do not stand out sufficiently to determine striking structural features. Of faults, two series are of great importance. The older of these has a general transverse Caledonian, north-westerly to south- easterly direction, and would seem to belong to the series of earth- movements which produced the cleavage of the district. Two of its members, one to the north and the other to the south of the summit of :M:oel Llyfnant, have produced notable displacements of the resistant rocks of the Volcanic Series, but appear to die out downwards in the soft shaly rocks of the Tremadoc and Dolgelly groups. That which determines the fine crags of u Castell, south of Llyn Arenig Fawr, appears actually to die out in the Daerfawr Shale, and cannot be proved to cut the Lower Volcanic Series at all. These faults I regard as tear-faults; they are very like the faults described by Dr. Marr among the Coniston Grits and older rocks of the Lake District, The rocks in their immediate neigh- bourhood are always much shattered; and it is interesting to note that the pieces of fault-rock are mostly slickensided in a direction which makes only a small angle with the bedding of the adjoining country rock. Parallel to these tear-faults are several fine examples of normal faults. The most important of these is the clean-cut fault which Downloaded from http://jgslegacy.lyellcollection.org/ at University of California-San Diego on January 17, 2016

634 ~R. w. G. VE~aXSlDES O~ XaE GEOLOGYOF ~Aug. I9O 5, passes close by Amnodd-wen, and gives the summit of Arenig its characteristic double peak. It has a throw of about 300 feet, and appears on the surface in turn as a gully and with fault- cliffs on either side successively. It seems to run right across the district, and is recognizable on both sides of the much later north- and-south fault; it has a notable hade to the north-east. Another important fault belonging to the same series is that which forms the waterfall at Pont Aberderfel. It passes close behind the Rhyd- y-fen Hotel, through Bwlch Llestri and on to Naat-yr-Achab. It is worthy of notice, because near Tai tterion it brings Asaphell~ts- Flags against Exte~sus-Flags and Calymene-Ashes, and, making there no feature, led the older geologists to record Arenig and Tremadoc fossils from the same locality.

The second great series of faults is represented only by one great example, which traverses the e2~tire district, and cuts the country and all its structures completely iu two. It was traced by Sir Andrew Ramsay in a direction a little west of north and east of south, for more than a dozen miles. It has a downthrow to the west, and at one place brings the Ashes of tho Upper Series against the Lower Dolgelly, or even the Ffestiniog Beds of the Lingula-Flag series. It is much later than all other structures in the district, and from tbe frequent stories of earth-rumblings which one hears at the farms along its course, one must conclude that it is probably still in a condition of instability.

In addition to these faults, there are also a couple of rather gentle anticlinal folds which affect outcrops in the Tryweryn Valley, and again in the neighbourhood of Llechwedd Erwent. These would seem to be somewhat earlier than even the oldest faulting, and pro- bably belong to the time of the intrusion of the andesitic dolerites. They have a Caledonian trend, and have taken a pitch corresponding to the general dip of the beds of the whole country. The more southerly example is cut off along its north-western flank by a fault. III. Co,~c~vslo3"s. This brings us to an end of our consideration of the solid geology of Arenig Fawr and Moel Llyfnant, and I may now briefly summarize the results which have been attained. The rocks are divisible into an Upper and a Lower Series, which are separated by an unconformity. The Lower Series belongs to the accepted Upper Cambrian rocks of North Wales, and includes representatives of the Ffestiniog, Dolgelly, and Tremadoc groups, the last of these being incomplete upwards. The Ffestiniog and Dolgelly Beds are quite like the corresponding beds of both the Tremadoe and the Dolgelly districts, while the character of tile Tremadoe is intermediate between that of the beds of Penmorfa and those of Sheinton, and is quite unlike that of the ashy beds described as Tremadoc at Dolgelly. Downloaded from http://jgslegacy.lyellcollection.org/ at University of California-San Diego on January 17, 2016

Vo]. 6I.] ARENIG FAWR AND ~iOEL LLYFNAI~T, 635

The Upper Series belongs to the Ordovician (Lower Silurian of some authors), and may be correlated division by division with the series described by Prof. Lapworth and Prof. Watts in the Shelve country. This series conveniently divides into a lower member of calcareous, gritty, or graptolite-bearing sediments, a middle member chiefly of direct volcanic origin, and an upper shaly member which is not described. The lower member is the equivalent of the Upper Skiddaw Slates of the Lake District, and is the Arenig of most British authors; whereas it was to the Middle Series that the name of Arenig was first applied by Sedgwick. The Volcanio Series is immediately overlain by beds belonging to the Caradoc or Bala Series, which are essentially Upper Ordovician in their fauna. Having thus stated the facts, I leave it to others to suggest a way out of the tangle in which our nomenclature has become involved, and shall hope to hear the matter fully thrashed out anon.

IV. GLXCI.~TIOX. Forming, as the Arenig Mountains do, the highest ground in the neighbourhood of the main watershed of Wales, their glacial geology is perhaps worthy of passing notice. Of the period of maximum glaciation there is little record, and it is uncertain whether or not the mountains were ever entirely overridden by ice. Certain it is, however, that the highest summits are so much frost-split, that they would not be likely to retain such strim as they might receive. No Drift has been found at higher levels thau 2000 feet. The stage when the ice stood at about 2000 feet is more interesting. Then and onwards, until it had retreated to about 1800 feet, all the important hilltops protruded as nunatakkr, and withstanding the powerful ice-stream which came along from Snow- donia and other districts to the west, retained numerous big boulders of a black-looking felspathic agglomerate like the Glan-y-pwll trap-rocks of Blaenau Ffestiniog or Maned. Perched blocks of this rock frequently occur quite high up on Moel Llyfnant, and are particularly abundant on the Lower Agglomerate slopes below Craig-yr-Hyrddod. They are quite distinct from any rock in the district. At this stage, the ice was able to supplement the local glaciation of the Blaen-Lliw Valley, and to send a weak lobe bearing a few Snowdoniau boulders into the Erwent Valley. At this stage, too, the ice dragging round the Daerfawr-Shale ground under Daerfawr, towards the open low ground of Nant Hir east of Arel~ig Fawr, began the formation of the lateral moraine, which ultimately formed the barrier that holds up Llyn Arenig Fawr, and similarly passing round Arenig Faeh to the Gelyn Valley also formed the barrier for Llyn Arenig Fach. At a later stage, the ice-lobe crossing the col between .-~Ioel Llyf- nant and Arenig Fawr became broken, and the part in the hollow to the north was left stagnant. In melting, this gave rise to a fine series of parallel ridges of morainic material, which, running Downloaded from http://jgslegacy.lyellcollection.org/ at University of California-San Diego on January 17, 2016

636 ~iv,. w. G. :FEARNSIDES ON THE GEOLOGY OF EAug. I905, roughly parallel to the sides of the cwm, enclose the morass between Ceunant-y-gareg-ddu and Amuodd-bwll. The retreat of the ice from the 1800- to the 1500~ level must have been comparatively rapid, and but little moraine can now be found between those limits. Below 1504) feet, however, moraines become abundant, and some of their terraces are exceedingly-well marked. One along the north side of the higher Tryweryn Valley is almost like a railway-embankment, and the same might also be said ibr that on the south-western flank of the Afon Tai-Herion Valley. Where these two meet, they have so overshot the outstand- ing crag of Craig Henllan that they enclose a triangular hollow, which (formerly a lake or turn) is being rapidly drained by a small outflow-stream, and is now only an impassable bog. The moraine which holds up Llyn Arenig Fawr is also mainly at this level. Unlike the moraines at 1800 or '2000 feet, those at 1400 to 1500 feet are almost wholly local in their origin ; and I am inclined to think that, at the time of their formation, the ice-shed lay much nearer the present watershed than at the earlier stage. The later glaciation, however, was no small phenomenon, for the Drift includes boulders of enormous dimensions; and though only the andesitic dolerites and intrusive andesites retain their scratches, all rocks within its boundaries are exceedingly-well moutonnds. A still later stage in the glaciation is marked by the accumula- tion of various tumps and roughly-crescentic mounds of Boulder-Clay and ill-sorted gravel in the valley-bottoms. One of these, occupying the upper end of the old valley-channel south of Foel Bodrenig, through which the railway now runs, was probably responsible for the diversion of the Afon Tryweryn over the beautiful falls of Boch-y-rhaidr, and the consequent formation of the great peaty flat above the falls. The origin of the lake of Tryweryn is not at all clear; but as it lies almost upon the watershed, is shallow, and has its exit over obvious Drift, we may reasonably conclude that it too is due to similar causes. No evidence of any glacial overthrow-valleys has been detected, but as the ice never advanced into any important valleys, this is hardly to be wondered at. The dip-slope valleys on the east of Areuig do not seem to have ever been seriously glaciated, and are much choked with frost-tahs.

In conclusion, I desire to express my best thanks to Dr. ~[. E. Marr, F.R.S., :P.G.S., for his constant good advice and encourage- ment in the working-out of the succession; to Miss Gertrude L. :Elles for her aid in identifying my graptolites; to Mr. R. H. Rastall, F.G.S., and all the various junior members of the Cambridge Geological School, who have shared and helped me to enjoy the glorious solitudes of Merionethshire; and to all the various land- owners, tenants, and hospitable farmers, who have allowed me to wander free over the country and i~nd out whatever new infor- mation is contained in this paper. My thanks are also due to the Director-General, and especially Downloaded from http://jgslegacy.lyellcollection.org/ at University of California-San Diego on January 17, 2016

Vol. 61.] AI~,XlG I:,~WR ,i.~D ~10Er, zrrv~.~,~-:r. 637 to Mr. H. H. Thomas, M.A., F.G.S., and other officers of the Geological Survey, at Jermyn Street, by whom the colour-model of the map was prepared.

EXPLANATION OF PLATE XLI. Geological map of Arenig Fawr and Moel Llyfnant. on the scale of 3 inches to the mile. The topography is based o~ the Ordnance-Survey map, and consequently, the mutation in the spellina~, of certain place-names, etc. difibrs occasionallyfrom that adopted in the text of this paper.

DiscussioN. The PRESIDENT said he was glad that the Author had turned his attention to Sedgwick's area.of typical Arenig rocks. The founders of our systems had been obliged to draw the lines between .different series of those systems somewhat vaguely, and it was desirable, lu the light of our increased knowledge of fossil-horizons, to ascertain what fossil-horizons were represented in these typical districts. The Author had availed himself of the information which Prof. Hughes and Mr. G. J. Williams had generously placed at his disposal, and was able, as the result of this and of his own work, to give us important information concerning the Lil,gula-Flags and Tremadoc Slates, as well as the Arenig rocks. With regard to the line between the Lingtda-Flags and the Tremadoc Slates, if the Dict.;io~tema-Bed were taken as the horizon of separation, the Psilocephah~s-Beds of Tremadoe, once placed ia the Tremadoc-Slate Series, must be assigned to the Li~,gvh~-Flags. It seemed desirable that this should be done. The occurrence of the Shineton fauna in Wales, originally proved by Williams in the Arenig district, had also been detected by the Author in that of Tremadoe, and he had largely added to our know- ledge of ~hose beds, which occurred below the well-known Angelina- Beds. The term 'Shu~,a~'dia-Beds' used by the Author seemed appropriate. Passing to the Arenig rocks of Arenig, the Author mentioned the occurrence of Loga~,ocqra2tus in his Evtenstts-Beds. It would be interesting to know whether the rich assemblage of LouanotT,'a2ti and Diclwgrapti found in Lakeland, Scandinavia, Canada, and Australia would be eventually discovered in these beds, in order to ascertain the nature of the earliest fauna of these Arenig rocks. The poverty of the fauna of the Daerfawr Shales was a matter for regret. If these shales were referable to the Murchisoni- horizon, the shales and the great mass of Upper Ashes were not of &renig, but of Llandeilo age. But there was still a difficulty. Where were the Upper Llandeilo Beds ? He (the speaker) had a dim recollection of having seen a specimen of Ogygia dilatata collected by Williams from shales above the ashes, but would not speak with certainty. But, so far as the tract described in the paper was concerned, the Author gave reasons for supposing a QUART. JOURN. GEOL, SOC. VOL, LXI, PL XLI.

[~ a~wqr ap tu~,-S h,ales D~fet ( Orthi~) Limestone

Upper (Rhyolitic) Ash

Middle Ash I i

Lower (Acid Andesitic) Ash ©

Daerfawr ( Didym~graptus- © Murvhisoni) Shales Lower ( A ndesitiv ) A st~s C~ IGNEOU~ Agglomerates of Arenig CONTEI~

Downloaded from http://jgslegacy.lyellcollection.org/ at University of Upper IUt!/~)litic Ash California-San Diego on January 17, 2016 Ohq~'a (Didymograptus- UPPER ASH ESt / b~fidus) 8h~s OF .] Middle Ash Filltirgerig ( Didy~wgraptus - A R ]i~'N IG hirundo) Beds [ L.wer (Acid Andesitic) Ash Erwent ( Ogyqia) Limestone

Henllan ( Caly~me) ..) Ua~careous Ashes J..u!er ( Andesiti, ) ,4.~he.~d! Aggl~)merate ~/" Arenig LlyJ~,ant (Zh:dymograptus - exten~) Flag, Basal Grit Hypersthene-.4 tute~te

Andesitic Dolerites Amnodd ( Shumardia ) Shales

Tai-herion ( AsapheUus) Flags Hornblende -l)orphy~tes Nant-ddu (Bellerophon) Beds Dictyonema- Band

Niobe-Beds z Peltura -.Beds Orthis-l~ticula'ris Band Parabolina -Beds $

Li~ula-Beds Gritty Lingula-Fla#s

8EMROS~ D£RBE 'T. SCALE : 3 INCHES := I ~ILE. Downloaded from http://jgslegacy.lyellcollection.org/ at University of California-San Diego on January 17, 2016

638 THE GEOLOGY Or ARENIG FAWR AND I~IOEL LLYFNANT. [Aug. I9o 5, passage between the Daerfawr Shales and the Upper Ashes, and between the latter and the Ortlds-Limestone. This limestone the Author referred to the Caradoc Series, and the fossils seemed to bear out his view ; but it was distinctly a surprise to find Caradoc rocks in that locality. Mr. A. 1). Young remarked on the unsettled use of the term laccolite, and wished to ask how far the structures studied by the Author resembled those intrusive masses to which the name in question was first applied. The floor of country-rock, the vaulted roof, and the absence of any cha~Jnel of communication with the surface, seem to be essential characters of a laccolite, as originally described. t)rof. HULL expressed his high appreciation of the geological mapping of the Author in the Arenig 5[ountains; and emphasized the great advantage in the prcsent day of having maps on the 6-inch scale, compared with those on the 1-inch scale (which were the only maps available in the old days of the Geological Survey, when the district was being mapped by his deceased colleagues, Ramsay, Selwyn, Jukes, and Aveline). It was under these able men that he (the speaker) had his first lessons in field-geology among these mountains, when he assisted in levelling the horizontal sections over Aran, Cader Idris, and . He believed that he was correct in saying that the view of the Government surveyors, as regarded the Arenig range, was that the great sheets of felspathic trap described by the Author were in a certain sense 'con- temporaneous,' and not 'intrusive,' except that they had been extruded from time to time over the floor of the sea; a view which the occurrence of great beds of volcanic ash with marine fossils tended "to confirm On the other hand, the dykes of diorite and dolerite, and other basic rocks which traversed the Arenig :Beds, were undoubtedly intrusive (in the ordinary sense) and of later age. Prof. WAr~s compared the succession at Arenig with that worked out by Prof. Lapworth in Shropshire. The main volcanic series was above the zone of Di(lymoyra2)tus Murchisoui in the former district, and below it in the latter. :But the chief members of the Arenig succession could be compared fairly closely. A strong resemblance also existed between the petrological characters of the andesites and bhe intrusive dolerites in the two areas, and there were similar difficulties as to the conditions of the outpouring or intrusion of the andesites. It was somewhat difficult to compare the Tremadoc rocks in the two localities. Prof. T. McK. ttvGnrs thought that the Society would allow that circumstances had laid the task of bringing before the Society the results of researches on the Arenig and associated rocks of the Arenig Mountain upon one well qualified by his stratigraphical, pal~eontological, and petrological skill to deal with it. He would only add a few remarks, on some of those points as to which there was less agreement. The Arenig of Sedgwick was defined to be the shales tangled among the volcanic rocks of the western front of Downloaded from http://jgslegacy.lyellcollection.org/ at University of California-San Diego on January 17, 2016

u 61.7 THE GEOLOGY OF AI~EXI6 FAWR AND MOEL LLY.FNANT. 639 the mountain. He had visited the district with Ramsay and Aveline when the difficulty of fixing a base first became acute, and as the grit so well seen between Portmadoc and Garthbarn seemed always to occur where the palceontological change from Tremadoc to Arenig set in, it was taken as the base. This was founded chiefly upon J.W. Salter's palmontological work, for in those days they had no detailed succession based upon the graptolites, such as :Miss Elles had now established. The chief difficulty about this base was, that although at Portmadoc the grit rested upon shales which contained A~geli~a within an inch of its base, at Garthbarn the grit was divided by a fossiliferous shale, and farther on towards Tan-y-bwlch it was represented by several lenticular beds of grit, so that some grits were well up in the Arenig, while the overlapping beds of the Arenig crept one over the other on to various parts of the uncon- formably-underlying Tremadoc. At the top of the Arenig also there was a difficulty as to the best place to draw the upper boundary, because the beds which crop2ed out farther west towards Ba]a (and had always been taken as Bala) reappeared on thc top of Arenig Mountain. As, however, the Arenig passed up into the overlying Bala, this was only a small matter of convenience of correlation. The question was asked, where then was the Llandeilo ? He pointed out that across the middle of the island there was evidence of an east-and-west movement, which had not had much effect on the beds up to the top of the Tremadoc, but had resulted in a different stratigraphical and pa]~eontological facies in the Arenig, Bala, and Silurian. In the south, a number of limestones occurred in the Silurian, without which the divisions would never have been drawn where they were; while the muddy and sandy sediment of the northern area differed much from that of the south. So in the Arenig and Bala Series a number of volcanic rocks occurred in the northern area, showing that the conditions were different there from those which prevailed in the south. The Llandeilo group was not absent owing to any unconformity or contemporary erosion, but simply because the conditions affecting sedimentation and life were different, and sufficient data were not yet available to determine which deposits were being laid down in the Arenig area when the Llandeilo :Flags of Caermarthenshire were being formed. Mr. P. LAKE drew attention to the similarity of the lower part of the Author's sequence and that of the Dolgelly district, de- scribed by Profi Reynolds and himself. Up to the Dictyonema-Beds the correspondence was precise, excepting for the fact that the Niobe-Beds had not yet been recognized at Dolgelly, probably on account of the poorness of the exposures. But the Bellerophon- and higher beds were apparently absent at Dolgelly, or were perhaps represented in part by the volcanic beds which appear a short distance above the Dictyone~a-Band. The most remarkable feature, how- ever, was that at Dolgelly, as in the Arenig district, the Tremadoc Beds were covered unconformably by laccolitic masses or sheet.s of doleritic rock. Prof. Reynolds and the speaker had inferred that these had been intruded along an unconformity ; but, as the overlying Downloaded from http://jgslegacy.lyellcollection.org/ at University of California-San Diego on January 17, 2016

640 ~tIE O~OrOOY or Ae.E~'IG r A~-D ~OEL r Lrrsa.N~. [Aug. I9o5, beds had been entirely removed by denudation in the area mapped, they were unable to determine the age of the unconformity. The Author's observations rendered it highly probable that it was the same unconformity as that which he had described at the base of the Arenig Series. In conclusion, the speaker desired to add his thanks to those of the previous speakers, for the very careful and valuable piece of work which the Author had brought before the Society. Mr. H. H. THor~xs congratulated the Author, on behalf of his colleague, Mr. T. C. Cantrill, and himself, on the excellence and general utility of his work; for it was always desirable to have a classical area competently revised, with the help of modern methods of research. He remarked on the striking similarity, both lithe- logical and faunal, between most of the members of the Arenig and Llandeilo rocks of the Arenig district and similar horizons in South Wales, over that part remapped by the Geological Survey in the past three years (Llandeilo to Whitland). He considered that in the two areas it would be quite possible to correlate the Arenig and Llandeilo rocks, bed for bed. In South Wales there was no uncon- formity at the base of the Arenig Series. The PRESIDENT said that there appeared to be differences of opinion on some points among previous speakers, as to matters of detail, but all were agreed as to the value of the paper, tie was glad to add his testimony on this, to that of the other Fellows who had taken part in the discussion. The Ae~rroR thanked the President and Fellows for their kind reception of his paper. In reply to the President, he suggested that the E.vte~sus-Flags included a representative of the highest part of the ' Many-branching graptolite '-fauna of the Lakes and elsewhere. With the President and Prof. Watts, he too was not satisfied as to the entire absence of the Upper Llandeilo :Beds. but had hunted for them in vain all over the district discussed. He remarked that Upper Llandeilo Shales were exceedingly well developed in the district west of Tremadoc and all along the north-western margin of Snowdonia, but even there were locally quite pinched out. He hoped to clear up the matter by future work in the interve,,ing districts. In reply to Mr. Young, he said that he had used the word laccolite to describe rather large cake-like masses of intrusive igneous rock, the general extent of which coincided with the bedding- planes of the neighbouring sediments, and the thickness of which was considerable (about ~-6) compared with their linear extent, tie particularly thanked Prof. Watts and Messrs. Lake and Thomas ibr their useful and valuable hints as to the correlation of the beds of the Arenig districb with those of other areas.